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Introduction
Earth has been called the green planet, a world clothed in a mantle of vegetation that sustains all other forms of life on this tiny spot in the universe. From simple beginnings, plants evolved first among Earth’s living things and thereby established a fundamental principle of nature: Plants, in one form or another, can exist forever without animals, but animals cannot exist without plants.

Plants purify the air by exchanging the oxygen we breathe with carbon dioxide, which is poisonous in too high a concentration. Plants convert the energy of sunlight into foods that sustain all animals and, from the soil, draw minerals such as nitrogen, potassium, calcium, and iron that are essential for our well-being. For creatures large and small, plants provide shade from the sun, refuge from predators, and protection from the ever-changing elements. Since the first cells came into being millions of years ago, plants have been the connecting links in an unbroken chain of life. It is they that have made the biosphere, the part of Earth’s crust where life exists, a place that humans can comfortably inhabit. The range of uses we make of plants is as broad as our ingenuity permits. We have exploited them for fibers to make cloth, drugs to cure a multitude of ailments, and wood to construct houses, furniture, and ships. From them we have extracted raw materials to manufacture innumerable goods, including paper. Without that latter commodity, our detailed history would not have been recorded and so remembered, nor could knowledge have been so easily disseminated. And culture, the possession of which makes humans out of animals, would never have developed beyond the basic skills and habits of primitive peoples had we not had paper on which to write music, poetry, and prose.

Some of us look at plants as a source of livelihood, while others find them intriguing subjects for scientific study. But most enjoy plants for the sheer delight of having them in their everyday surroundings, to savor the varied colors, textures, tastes, and aromas that they alone can offer. Few gardeners share the botanist’s knowledge of plant biochemistry, anatomy, physiology, and intricate reproductive systems, yet all have experienced the extraordinary satisfaction derived from growing flowers, fruits, vegetables, and trees.

When we work with plants, questions about them inevitably come to mind. What takes place inside a seed after we have set it in the ground? How does water travel from soil to treetops? What makes a plant become bushy with repeated pruning? What controls seasonal flowering patterns? How do plants grow, and why is light necessary to make growth happen? Over the centuries, botanists have worked to find answers to these and other questions. Slowly, plants have revealed some of their secrets.

Botany is a useful and rewarding study from which, unfortunately, many laypersons are frightened away by the technical jargon that constitutes the language of the science. The reader will encounter scientific words in the following pages. Some are part of the common parlance of gardeners. For want of suitable nontechnical equivalents, others cannot be avoided when writing such a book. Each technical word, whether common or obscure, is explained in the text and glossary, and occasional reference is made to the Greek and Latin roots from which these words have been derived. In addition, the numerous illustrations give added meaning to the botanical vocabulary and ideas developed.

Some of the photographic subjects are not the customary things that gardeners look for in plants, but they are plants or parts of them seen in close-up, sometimes through a microscope. A majority of the plant specimens that have been photographed were selected from those available in my own and neighbors’ gardens, local parks, and botanical gardens in southern California. But the broad principles of botany each photograph exemplifies are equally applicable to plants in almost any part of the world.

There are close to 400,000 recognizably different kinds of plants, or species, in the world today. So diverse are their forms that to write an all-inclusive definition of the word plant is not at all easy. One-third of all plants do not have roots, stems, and leaves as we know these parts in the examples most familiar to us. About 150,000 plant species never produce flowers, and almost that same number do not grow from seeds, but rather from dustlike particles called spores. The vast majority of plants manufacture their own food supplies by a process called photosynthesis. Mushrooms, molds, and other fungi rely on foods created by green plants for their sustenance (as do animals) and, for that reason, have now been reassigned from the plant kingdom to the fungal kingdom. Most plants spend a lifetime anchored in one place, yet a few simple, one-celled plantlike organisms are capable of swimming to different locations in the waters they inhabit. It is this kind of diversity and amazing variety of shapes, colors, and lifestyles that continually excite our interest in these organisms called plants.

As we delve into the science of botany, we shall largely be concerned with the two groups of plants with which we, as gardeners, most often work. One, known as the flowering plants, or angiosperms, is the largest group in the plant kingdom and consists of about 250,000 species. The name angiosperm refers to the fact that seeds from these plants are formed inside containers that we call fruits (Greek: angeion, “vessel”; sperma, “seed”). The flowering plants most often decorate our homes and landscapes, supply almost all of the vegetable matter in our diets, and are the source of the world’s hardwoods. They are the most sophisticated of plant forms and are best adapted to survive in a wide range of climates and places.

Second are gymnosperms, plants that produce seeds in the open spaces of cones—between the flaplike parts that make up a pine cone, for example. The Greek words gymnos, “naked,” and sperma, “seed” describe this form of development. On the evolutionary scale, gymnosperms are more primitive than angiosperms but are of considerable economic importance as well as interest to landscapers for their compact forms and richly colored, needle-shaped, or scalelike leaves. Softwoods such as pine and fir are not only used to make paper, lumber, and plywood, but are the source of utilitarian products such as pitch, turpentine, and rosin. The gymnosperms include all the conifers: cedar, redwood, juniper, cypress, fir, pine, and the largest living things on earth, the giant sequoias. Members of this group include many ornamental shrubs, such as varieties of Chamaecyparis (false cypress) and Thuja occidentalis (American arborvitae); the beautiful maidenhair tree, Ginkgo biloba, a broad-leaved species; and, the least typical of gymnosperms, the cycads.

For comparative purposes, passing mention is made of ferns, mosses, and other primitive plants, but it is to the flowering plants and gymnosperms that we direct our attention because it is they that give us the most revealing picture of how marvelous plants are.